1. Field of the Invention
This invention relates to a photothermographic element and in particular, it relates to a photothermographic element containing core-shell-type silver halide grains.
2. Background to the Art
Silver halide containing photothermographic imaging materials (i.e., heat-developable photographic elements) processed with heat, and without liquid development, have been known in the art for many years. These materials, also known as "dry silver" compositions or emulsions, generally comprise a support having coated thereon: (1) a photosensitive material that generates elemental silver when irradiated; (2) a non-photosensitive, reducible silver source; 3) a reducing agent for the non-photosensitive reducible silver source; and (4) a binder. The photosensitive material is generally photographic silver halide which must be in catalytic proximity to the non-photosensitive, reducible silver source. Catalytic proximity requires an intimate physical association of these two materials so that when silver specks or nuclei are generated by the irradiation or light exposure of the photographic silver halide, those nuclei are able to catalyze the reduction of the reducible silver source. It has long been understood that elemental silver (Ag.degree.) is a catalyst for the reduction of silver ions, and the photosensitive photographic silver halide may be placed into catalytic proximity with the non-photosensitive, reducible silver source in a number of different fashions, such as by partial metathasis of the reducible silver source with a halogen-containing source (see, for example, U.S. Pat. No. 3,457,075); coprecipitation of silver halide and reducible silver source material (see, for example, U.S. Pat. No. 3,839,049); and other methods that intimately associate the photosensitive photographic silver halide and the non-photosensitive, reducible silver source.
In both photographic and photothermographic emulsions, exposure of the photographic silver halide to light produces small clusters of silver atoms (Ag.degree.). The imagewise distribution of these clusters is known in the art as a latent image. This latent image generally is not visible by ordinary means and the photosensitive emulsion must be further processed in order to produce a visible image. The visible image is produced by the reduction of silver ions, which are in catalytic proximity to silver halide grains beating the clusters of silver atoms, i.e. the latent image. This produces a black-and-white image.
The non-photosensitive, reducible silver source is a material that contains silver ions. The preferred non-photosensitive, reducible silver source comprises silver salts of long chain aliphatic carboxylic acids, typically having from 10 to 30 carbon atoms. The silver salt of behenic acid or mixtures of acids of similar molecular weight are generally used. Salts of other organic acids or other organic materials, such as silver imidazolates, have been proposed, and U.S. Pat. No. 4,260,677 discloses the use of complexes of inorganic or organic silver salts as non-photosensitive, reducible silver sources.
As the visible image is produced entirely by elemental silver (Ag.degree.), one cannot readily decrease the amount of silver in the emulsion without reducing the maximum image density. However, reduction of the amount of silver is often desirable in order to reduce the cost of raw materials used in the emulsion.
One method of attempting to increase the maximum image density in black-and-white photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating toning agents into the emulsion. Toning agents improve the color of the silver image of the photothermographic emulsions, as described in U.S. Pat. Nos. 3,846,136; 3,994,732; and 4,021,249.
Another method of increasing the maximum image density of photographic and photothermographic emulsions without increasing the amount of silver in the emulsion layer is by incorporating dye-forming materials in the emulsion. For example, color images can be formed by incorporation of leuco dyes into the emulsion. Leuco dyes are the reduced form of a color-bearing dye. Upon imaging, the leuco dye is oxidized, and the color-bearing dye and a reduced silver image are simultaneously formed in the exposed region. In this way a dye enhanced silver image can be produced, as shown, for example, in U.S. Pat. Nos. 3,531,286; 4,187,108; 4,426,441; 4,374,921; and 4,460,681.
Multicolor photothermographic imaging articles typically comprise two or more monocolor-forming emulsion layers (often each emulsion layer comprises a set of bilayers containing the color-forming reactants) maintained distinct from each other by barrier layers. The barrier layer overlaying one photosensitive, photothermographic emulsion layer typically is insoluble in the solvent of the next photosensitive, photothermographic emulsion layer. Photothermographic articles having at least 2 or 3 distinct color-forming emulsion layers are disclosed in U.S. Pat. Nos. 4,021,240 and 4,460,681. Various methods to produce dye images and multicolor images with photographic color couplers and leuco dyes are well known in the art as represented by U.S. Pat. Nos. 4,022,617; 3,531,286; 3,180,731; 3,761,270; 4,460,681; 4,883,747; and Research Disclosure, March 1989, item 29963.
With the increased availability of low-irradiance light sources such as light emitting diodes (LED), cathode ray tubes (CRT), and semi-conductor laser diodes, have come efforts to produce high-speed, photothermographic elements which require shorter exposure times. Such photothermographic systems would find use in, for example, conventional black-and-white or color photothermography, in electronically-generated black-and-white or color hardcopy recording, in graphic arts laser recording, for medical diagnostic laser imaging, in digital color proofing, and in other applications.
Various techniques are typically employed to try and gain higher sensitivity in a photothermographic material. These techniques center around making the silver halide crystals' latent image centers more efficient such as by introducing imperfections into the crystal lattice or by chemical sensitization of the silver halide grains and by improving the sensitivity to particular wavelengths of light by formulating new improved sensitizing dyes or by the use of supersensitizers.
In efforts to make more sensitive photothermographic materials, one of the most difficult parameters to maintain at a very low level is the various types of fog or D.sub.min. Fog is spurious image density which appears in non-developmentally sensitized areas of the element and is often reported in sensitometric results as D.sub.min. Photothermographic emulsions, in a manner similar to photographic emulsions and other light-sensitive systems, tend to suffer from fog.
Traditionally, photothermographic materials have suffered from fog upon coating. The fog level of freshly prepared photothermographic elements will be referred to herein as initial fog or initial D.sub.min.
In addition, the fog level of photothermographic elements often rises as the material is stored, or "ages." This type of fog will be referred to herein as shelf-aging fog. Adding to the difficulty of fog control on shelf-aging is the fact that the developer is incorporated in the photothermographic element. This is not the case in most silver halide photographic systems. A great amount of work has been done to improve the shelf-life characteristics of photothermographic materials.
A third type of fog in photothermographic systems results from the instability of the image after processing. The photoactive silver halide still present in the developed image may continue to catalyze formation of metallic silver (known as "silver print-out") during room light handling or post-processing exposure such as in graphic arts contact frames. Thus, there is a need for post-processing stabilization of photothermographic materials.
Without having acceptable resistance to fog, a commercially useful material is difficult to prepare. Various techniques have been employed to improve sensitivity and maintain resistance to fog.
U.S. Pat. No. 4,212,937 describes the use of a nitrogen-containing organic base in combination with a halogen molecule or an organic haloamide to improve storage stability and sensitivity.
Japanese Patent Kokai 61-129642, published Jun. 17, 1986, describes the use of halogenated compounds to reduce fog in color-forming photothermographic emulsions. These compounds include acetophenones such as phenyl(.alpha.,.alpha.-dibromobenzyl)ketone.
U.S. Pat. No. 4,152,160 describes the use of carboxylic acids, such as benzoic acids and phthalic acids, in photothermographic elements. These acids are used as antifoggants.
U.S. Pat. No. 3,589,903 describes the use of small amounts of mercuric ion in photothermographic silver halide emulsions to improve speed and aging stability.
U.S. Pat. No. 4,784,939 describes the use of benzoic acid compounds of a defined formula to reduce fog and to improve the storage stability of silver halide photothermographic emulsions. The addition of halogen molecules to the emulsions are also described as improving fog and stability.
U.S. Pat. No. 5,064,753 discloses a thermally-developable, photographic material containing core-shell-type silver halide grains that contain a total of 4-40 mole % of silver iodide and which have a lower silver iodide content in the shell than in the core. Incorporating silver iodide into the silver halide crystal in amounts greater than 4 mole % is reported to result in increased photosensitivity and reduced D.sub.min. The silver halide itself is the primary component reduced to silver metal during development. The shelf stability properties of the preferred formulations is not addressed. This material is primarily used for color applications.
Japan Patent Kokai 63-300,234, published Dec. 7, 1988, discloses a heat-developable, photosensitive material containing a photosensitive silver halide, a reducing agent, and a binder. The photosensitive silver halide has a silver iodide content of 0.1-40 tool % and a core/shell grain structure. The photosensitive silver halide grains are further sensitized with gold. The material is reported to afford constructions with good sensitivity and low fog.
Japan Kokai 62-103,634, published May, 14, 1987; Japan Kokai 62-150,240, published Jul. 4, 1987; and Japan Kokai 62-229,241, published Oct. 8, 1987, describe heat-developable photosensitive materials incorporating core-shell grains with an overall iodide content greater that 4 mol %.
U.S. Pat. No. 5,028,523 discloses radiation-sensitive, thermally-developable imaging elements comprising; a photosensitive silver halide; a light-insensitive silver salt oxidizing agent; a reducing agent for silver ion; and an antifoggant or speed enhancing compound comprising hydrobromic acid salts of nitrogen-containing heterocyclic compounds which are further associated with a pair of bromine atoms. These antifoggants are reported to be effective in reducing spurious background image density.